Thinking outside the box at Pratt & Whitney

June 16, 2016, © Leeham Co.: “I’m working on six or eight engines. The more the better.”

This startling statement comes from Alan Epstein, vice president of technology and the environment for Pratt & Whitney.

It runs counter to everything the airline industry has believed since the introduction of the twin-engine Boeing 767 was qualified for ETOPS in the early 1980s: fewer engines are better.

Epstein explained his statement during an interview with LNC at the United Technologies Media Days last week in Hartford (CT).

Epstein last year at the same event told LNC he was looking at four-engine technology for future airplanes. We asked him this year if he was still looking at four engines. That’s when he said he was looking at six.

It’s his job to think outside the box. This one clearly qualifies.

“In the mid-1990s, the engine people were whining that engines had become a commodity,” Epstein explained. “On the [Boeing] Triple 7, you could buy a Rolls or a Pratt or a GE engine. There wasn’t much to differentiate them except price. It’s a commodity. The airplane seemed to have the same capability. That was an anathema to the engine companies.

“People attacked it different ways,” Epstein said. “GE went and bought exclusives. They did it with money. They put big money up front. UTC decided to do it with technology. The GTF was a differentiator. Technology changed the business.”

Epstein said that in 2007 he discovered there was a paper in 1957 that laid out the case, “Why not use small engines?” rather than one big engine.

“I thought, why not examine their hypotheses and assumptions and see what’s changed in 50 years,” he said.

Epstein said engine accessories in 1957 didn’t scale, but they do today; electronic fuel control systems, for example. Engines also become lighter when they are smaller.

“When I gave a lecture on this, someone came up and said you can’t use a large number of engines. Do you realize how much time it will take just to check the oil?” Epstein said that today cars have an oil indicator and no dip sticks. “If my car can check the oil automatically, my airplane can check it automatically.”

Because engines are commodities, he said smaller, lighter engines in greater numbers on airplanes means an aircraft could be dispatched with one of many engines inoperative. Diversions would also be reduced, although he concedes that the engines today are incredibly reliable.

From an economic point of view, Epstein said, the engine shut down rate in the 1960s was 500 per million flight hours. The V2500s today are less than one in a million.

Smaller engines might invite new entrants into the field. It would also mean a company like PW could build hundreds of thousands of engines a year.

“It would be a disruptive technology to have that kind of propulsion system,” Epstein said. If the incumbent OEMs didn’t do it, some new entrant might, he said.

Read more of our interview with Alan Epstein behind our paywall today.

36 Comments on “Thinking outside the box at Pratt & Whitney

  1. “It runs counter to everything the airline industry has believed since the introduction of the twin-engine Boeing 767 was qualified for ETOPS in the early 1980s: fewer engines are better.”

    For history sake: in 1977, the A300B4 became the first ETOPS compliant aircraft, qualifying for Extended Twin Engine Operations over water, operating in Asia. Alarmed by the success of the A300, Boeing responded with the new Boeing 767 for replacing 4 engine 707s.

    The more engines trade-offs have never been away completely. I often wondered if the assumption all engines on an aircraft have to be identical, also had its trade-off.

    Contrary to what most believed the concept “Ecoliner” was a trijet, with a ~35k lbs engine in the tail to get “one engine out” take-off above the required 150 klbs. Idea was to rather use “regular” 115lbs engine instead of a new risky 150k lbs design.

    Less known is a similar tri engine Euro MPA for getting the heavy loaded airframe off the ground from hot short runways without oversized engines/wings..

  2. Interesting.

    I wonder where distributed propulsors figure into this 4 or 6 engine future he envisages. If turbines are used primarily to generate electrical power, rather than mechanical (turning the fan), then many long held optimisations may no longer be relevant.

  3. The “twin engine is best” is the result of underlying technology. It is a heuristic that fits to observed technology trends from 1960-2010. It is no physical law, actually if aircraft performance (fuel burn) is used as sole metric, the four engine aircraft usually wins (four engines weigh less than two as you need less thrust).

    It is when you add factors such as maintenance and operations things change in favor of the twin. Very hard to analyze.

    The most physical advantage is the better SFC coming from relatively smaller tip clearances of large engines, but I assume that this advantage becomes increasingly insignificant above a certain thrust range. Here we come in contact with scalability of technology, and Mr. Epstein may have a point.

    • “actually if aircraft performance (fuel burn) is used as sole metric, the four engine aircraft usually wins (four engines weigh less than two as you need less thrust).”

      so why was the A340 500/600 such a pig compared to the 777-300ER? same gen engines, same gen airframe….

      • Engine choice, they picked a bad engine setup for it

        Rather than optimized for 4 it was optima for 2 and a different application.

        The is a previous article on this by Bjorn that should be listed in the preamble as it were

        I was not a believer but I am now.

        Also a bit of history. Airbus came out with the A300, while it did not meet ETOPS USA, it did not bother the Asians (that’s where I flew one).

        Boeing saw the writing on the wall and lost market and came up with the strategy of getting ETOPS for 2 and the 767 was the result.

        Its now been adapted by all including singles aisle flights to Hawaii by several airlines which from a 4 engine guy from the 50s is unsettling.

        Reality is that 4 engines did not save anyone (sans WWII combat) and 4 engine aircraft disappeared on over water flights never to be heard of (MH370 is not alone, just popular)

        • Well, 4 engines does give options in unusual situations.

          BA Flight 9, the 747 that stumbled into a volcanic dust cloud over Indonesia in the night in 1982 survived because one of the 4 engines eventually came back to life, giving them time to get the others back online.

          If you’ve lost all engines, having a chance of getting 1 of 4 started is better than the chance of getting 1 of 2 running again.

          • Unlike a twin engine the 747 could not maintain level flight on one engine, it wasn’t until they got the second engine started that they were able to arrest their descent. So the odds of starting 2 out of 4 engines compared to starting 1 of 2…

          • Sure, but that one engine bought them time. I’m pretty sure that crew concluded that 4 were better than 2 that night.

          • I don’t view it as a safety factor. More engines, more likely to have one quit as well.

            Its going to hinge on if its a better more efficient combination overall if the 3 or 4 (or 6 or 8!) come back.

            I think we also have to update the reality.

            Volcano monitoring has hugely improved.

            We had a 747 go through a cloud back in the 90s, almost went down (all 4 engine quit)

            Now they have radar tuned to it, ash fall forecasts, wind direction and the ability to avoid or operating in it.

            JAL had a 747 with 4 engines almost crash going into Japan. fuel starvation. Last engine quit as they touched down.

            One funny one was that the DC-6 cargo planes up here did fine.

            Mechanic remembered that they had the original oil bath filers stashed on the premises. Got them out and away they went.

            Jets and Turbos were grounded.

        • “the A300, while it did not meet ETOPS USA”

          Here comes the Europeans with the worlds first twin-engine widebody, a potentially marked disruptor. American jobs could be lost… The USA then won’t approve twin-engine operations that isn’t very close to an airport. A good way of using “safety” as a means to reduce competition. When Boeing had the 767 ready, then of course it was ok, but not that far away from an airport so the 747 marked could be protected.

          Same with the first generation of 777. In order to increase 777 competitiveness against the A340, a new category ETOPS was invented, that is ETOPS 180. Later this was again extended to 240 minutes when it was clear that the 747 would have a hard time competing with the A380.

          And of course, in this context, let us also mention the ban on all civilian supersonic aircraft in the US. The Europeans made a passenger aircraft that can fly higher, faster, and over much greater distance then our best military fighters (at that time). After a long time, we had to acknowledge that supersonic flight over international waters couldn’t be banned. But the damage was already enough. Concorde be gone…

          • “And of course, in this context, let us also mention the ban on all civilian supersonic aircraft in the US. ” I think you can lay that one squarely at the feet of the environmentalists.

          • Here comes the “it’s an evil American plot” conspiracy ad nauseam…

          • Concorde was maybe “better” than the best US (and everyone else’s) fighters, but it wasn’t better than the A12/SR71 which flew into service earlier, higher and faster but did need a splash/dash inflight refueling to equal Concorde’s range.

            I read an account somewhere of an SR71 coming across a Concorde and forming up alongside for a while. The SR pilot commented on the amazing experience of being ensconced in a pressure suit in a cramped cockpit whilst looking out the window at passengers sat in the shirt sleeves, all of whom were looking back and toasting him with champagne! Concorde was slower and lower, but equally cool!

            Of course the A12 and SR71 both regularly flew supersonic over the USA, but only in very restricted corridors and not to the universal acclaim of the populace.

          • Of course Boeing went for it, its all about maneuvering for best advantage.

            Airbus started it and it escalated.

            At the time of conception fuel was not an issue nor was the environment nearly that high and certainly aviation issues in that regard.

            Concord was a technical marvel (and my hat is off to both the French and British for brilliant execution)

            Commercially it was a duck and only the aviation structure at the time allowed it to continue.

            Sadly the crash in Paris was its final hoorah, the loss of life was awful for such a fine aircraft to go out on.

            As Boeing found out with the Sonic Cruiser proposal, speed no longer pays for itself.

            If the US had the SST, it might have been different but the environmental movement was strong and it may have suffered the same fate. Occasionally corporation n don’t get their way.

            You might also consider that Europe ahs the US to thank for their clean air. I saw the pictures back in the 70s, it was awful (friends went their). All the cities looked like LA.

            US started the movement and its spread to where now Europe is a leader in a lot of ways (VW aside).

      • Why was the A340 such a “pig”…it is called hysteria. Read the Leeham study of the cost comparisons between 777 and 340. In reality, it depended on the mission mix, but they conclude that the real running costs were even.
        Airbus, for reasons unknown, did not challenge the A340 perception, but allowed the extra row 777’s to be compared to standard 8 A340’s.

  4. Check the dissertation by Richard Avellán from 2011. It has several charts where four engines come out with lower weight than three or two.

    The full text is available here:

    Link to dissertation

    Björn: this is a guy you might want to talk to. Let me know if you want contact details.

  5. I’m kinda curious about the one in a million engine shutdown rate. Looking at the reports on AvHerald it seems to be quite frequent that an engine has to be shut down.

    Are those data more like calculated statistical data like the one in 10,000 years earthquake that a nuclear power plant has to cope with or are these numbers really from recent flight stats?

    • Just to get an idea what to expect:
      There are ~3900 A320 active (6600 for the family )
      Lets assume an utilization of 8h per day.

      That is 1 Million flight hours every 32 days. ( family: 18 days )

      While engine reliability has increased over serveral magnitudes
      the same is valid for active fleet sizes.

  6. So, instead of four equal thrust engines,
    might there be some advantages to having
    two higher thrust inboard engines and
    two lower thrust outboard “helper” engines?

    • I have always been in the hope Airbus may discover the benefits from an asymetric propulsion, for a resonnable A380 “Cachalot” up tu 7000 nm & under 525 T MTOW, and with the help of a smaller wing, governs, and MLG Too !

  7. So maybe we will see an A380 with six Pratt & Whitney GTF engines? The same version as used on the A321neo…

    That would be the day. 🙂

      • Aren´t P+W looking at re-engining some of the B-52 fleet for the USAF? Talk was if it ever comes to anything it will be 8 x approx 30klb as cost saving of fitting larger engines doesn´t justify cost of new pylon etc. Maybe it will be a test case for 8 is better than 2?

        • You may remember that they looked at re-engining them with RR years ago, but concluded that they didn’t really bring any real benefits.

          Presumably the B52 has adequate range as is, and even more range wouldn’t bring worthwhile operational benefits. It’s combat radius is already 1.5 x Atlantic. And those TF33s are probably simple enough that literally anyone could keep them in service indefinitely equipped with nothing more than a half inch wrench, a wire brush and a few tins of recycled car engine oil…

          Essentially that was the conclusion they reached last time, and I’m not sure that anything has changed recently to make them look for an uplift in operational range.

          • I have yet to figure out the real reasoning on the B-52.

            I doubt the RR conversion, rings no bells.

            They redid the KC135 fleet (the part still flying) with CFM.

            The only reason they did not do all the 707 based airframes was fan interfere issues on the various spy planes (those I believe are upgraded P&W)

            I have not seen a credible explanation other than there is not enough political will by the USAF to do it (or priority)

            Huge fuel savings, those old engines break down a lot, more power etc.

            They re-engined (or are) the C5.

            Of all the US armed forces, I trust the USAF the least for rational thinking as opposed to self serving stove piped empire building.

            they want to retire the irreplaceable A-10. Dumb

          • I think the idea was to return some stored airframes back to service. The B-52s in use are pretty high in hours for a 1950s design. The earlier stored frames still have the original engines, as I undertood it. The article I read said they considered a program a few years ago to ust RR Trent but the cost of new pylons etc made it too expensive.

            Remenber there are studies going on all the time about all kinds of ideas, it will probably never happen, but P+W are being paid to look at it.

          • Danged if you aren’t right. I had not read that part.

            Can’t figure out why RR, very Un-American of them!

            the choices are very restricted now as both the P&W 2000 and the RR RB211 are out of producing (go used?)

            Now its 8 x GE CTF-34 or 4 x P&W GTF!

            Seems the GTF would be the better choice.

    • It might be 8 , B52 style. Even more savings by having a single pylon share 2 engines
      You need a special version of the shorthaul engine to do long haul flights. CFM did that for the A340 but could only do so much and dint quite make it.
      A fully optimised small GTF for long haul only would be very interesting

    • Trent 900 (A380) : 80,000 lbf
      PW1135G (321) : 35,000 lbf
      So it would take at least 8 upgraded GTFs to power the A380.
      GE90X (777X) :105,000 lbf
      That would be exactly 6 GTFs for a 777X!
      So if Epstein is talking about 6 engines it must be the 777X and probably also A350 (or an A340NEO?), not the A380. I believe that the plan for the A380 is already firmed, and that can only be 4 RR Ultrafans.

  8. Is he saying this because GTF engines does not scale well compared to traditional TurboFans?

  9. Hi Scott, Björn,

    Would you by any chance know the reference of the 1957 paper that is mentioned ?

    • Keesje, why don’t you show us an A350 and 777 with 6 GTFs each?

  10. Nasa has announce the future develoment of a piloted electric powered plane the X57 Maxwell.
    It will be a rewinged and re-engined P2006T Tecnan. There will be 6 propeller motors on EACH wing which will be used for takeoff and landing . While thrust during cruise will be provided by two larger engines on each wingtip

    6 motors on each wing plus one on each wingtip!

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